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62 Answers
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SharePoint Manager is a SharePoint object model explorer. It enables you to browse every site on the local farm and view every property. It also enables you to change the properties (at your own risk). This is a very powerfull tool for developers that like to know what the SharePoint holds of secrets.

I've found .NET Reflector (now from RedGate) to be an indispensable tool when developing, and with SharePoint in particular. In some cases it has saved me from spinning wheels when it is clearly a "bug" or "feature" in the SP codebase, but it most cases it has just been invaluable from a learning perspective (oh, that's how that works!)

SPTraceView is awesome for seeing errors that occur on SharePoint server, especially if it's your dev box as they happen via the System Tray! Also when you double click on tray it shows the exceptions...rather than you opening whole ugly ULS log ;-)

Sure fiddler is good, but Visual RoundTrip Analyzer (VRTA) gives you a nice graphical illustration and can also help you with advice in solving the problems.
It has issues regarding virtual environments though, but a great tool never the less (thanx Spence for introducing it to me:-) download VRTA

The content deployment wizard (http://spdeploymentwizard.codeplex.com/ ):
The SharePoint Content Deployment Wizard is a tool for SharePoint 2007 which provides the means to deploy the following content:

Microsoft Live Mesh is a great tool to handle all your other tools. I've set up a folder that I have all my tools in and then use Live Mesh to have these tools synchronized into my VM's and other machines. As soon as I add or update a tool, they are all "copied" to my other machines.

What? Noone said SQL Server Profiler yet? How the heck do you guys find out what goes on behind the scenes without it? :-)

Especially good when you are after better performance. A quick profiler run will find those pesky SharePoint "specialities" in .NET like using foreach instead of for loops, or the difference in performance of doing SPContext.Current.List.ItemCount and SPContext.Current.List.Items.Count, finding deadlocks, timeouts etc

Ehm, noone said MetaMan yet? Doint BDC by hand is such a pain... MetaMan is the best tool out there. And now also web based. (ok Nick you can cough up a with a dev license now ;-)
Nah seriously, its an expensive tool, and the licensing model sux (for devs at consultancy companies that has lots of different customers) but the tool is top notch and saved me more times than i care to remember from doing tedious XML

sharegate is a light yet powerful tool
to compare and synchronize your
SharePoint environments in only three
steps. It quickly detects the
differences between your sites and
allows you to send your chosen
selections to your desired destination
from anywhere in your organization.
Our tool accelerates your SharePoint
deployments by significantly reducing
the time consuming manual process. The
power of sharegate resides in its
simplicity and its cost. We all agree
that we need a simple tool for
SharePoint synchronization.